There are various recognized methods for dyeing and/or incorporating additives into fibers and other fibrous materials and particularly difficult to dye fibers such as aromatic polyamides, polybenzimidazoles and polyimides. These methods sometime utilize organic solvents such as dimethylsulfoxide ("DMSO"), dimethylacetamide ("DMAC"), dimethylformamide ("DMF"), N-methylpyrrolidone ("NMP"), N-cyclohexyl-2-pyrrolidone ("CHP"), acetophenone, benzyl alcohol, acetanilide and methyl benzoate. Exemplary systems utilizing these organic solvents have been proposed in U.S. Pat. Nos. 3,558,267 to Langenfeld, 3,741,719 to Ramanathan et al, 4,525,168 to Kelly, 4,898,596 to Riggins et al and U.K. Pat. Nos. 1,275,459 to Gruen and 1,282,113 to Burtonshaw et al. the disclosures of which are incorporated herein in their entirety. Other methods are described, for example, in U.S. Pat. Nos. 4,780,105 to White et al., 5,092,924 to Neely et al. and 5,207,803 to Holsten et al. the disclosures of which are incorporated by reference herein in their entirety.
The organic solvents are sometimes referred to as "swelling agents" in that solvents swell the fiber at elevated temperatures to permit the dye molecules to be introduced into the fiber. It is believed that when the swollen fiber is cooled or the solvent is removed (e.g., by heating), the fiber returns to its original condition and the dye molecules are entrapped in the fiber. Certain of the organic solvent swelling agents, however, tend to damage the fibers by the swelling action. For example, organic solvents can adversely affect tear strength properties. There also has been some environmental concern with respect to certain of the organic solvents.
Retention of solvent can adversely affect both the appearance and the flame resistance of the fibers. Prior art techniques for dyeing difficult to dye fibers such as aromatic polyamides (i.e., poly-m-phenyleneisophthalamide) ("Nomex.RTM.") and poly(p-phenyleneterephthalamide) ("Kevlar.RTM."), polybenzimidazoles, aromatic polyimides, and the like can be counter-productive in that one of the problems caused by retention of solvent (e.g., decreased flame resistance) requires addition of a significantly large portion of a flame retardant to compensate for the reduction in the flame resistant properties of the fibers.
It is therefore an object of the present invention to provide an improved method and dye assistant for dyeing fibrous materials, and particularly to provide a method and dye assistant which obviates the need to utilize organic solvents and carriers that tend to damage the fibers by their swelling action.
It is another object of the present invention to provide a method and dye assistant having one or more particularly advantageous results, including increased efficiency in dye exhaustion, maintenance of tear strength, and more level and uniform dyeing as compared to the absence of a carrier or organic solvent.
The improved method and dye assistant is particularly advantageous when used with difficult to dye fibers such as aromatic polyamide fibers (e.g., Nomex.RTM. and Kevlar.RTM. fibers), polybenzimidazole fibers, aromatic polyimide fibers, fibers of copolymers of the monomers thereof and blends thereof.